Manufacture of Boiled Salts.—For fine, superfine, butter-, and eheese-salts, the pan employed is not more than 30-35 ft. long, sometimes as low as 25-26 ft., by 22-25 ft. wide. The salt is fished out of these pans twice each day, perhaps a little oftener for butter-salt, and the pans have to be laid off and sealed every month, or even oftener. These boiled salts are all fine grained, some rather finer than others. This fineness is obtained by keeping the brine not only boiling, but well agitated by raking. Superfine salt is but the same rather more carefully made, and subsequently ground. Butter- and cheese-salts, which hardly differ from each other, are not stoved. The salt from the boiled-salt pans is drawn to the sides of the pan by iron rakes, when it is lifted out of the brine by means of perforated shovels, called " scoops," the ends of which are turned up for this purpose, as shown at C, Fig. 1204. Two forms of drainers are used in Cheshire, one conical, called a round tub or basket, the other, already mentioned, called kb square tub. The former vary rather in eapacity, but usually they are about 3 ft. from the brim to the end of the foot. Square tubs are 18 in. long by 7 in. sq. at one end and 9 in. sq. at the other end, consequently they form truncated quadrilateral cones. Their bottoms are removable. For the purpose of filling, the round tubs are ranged, standing on their pointed ends in the brine, around the inner sides of the pan, and as the salt is fished up, they are filled with it one after the other. The square tubs are filled in the same manner, only where these are used, the pan is surrounded with a sort of gallery or platform of bar-iron, on the inside, just barely below the surface of the brine, on which the tubs are placed to be filled. They are then carried to the stove-room, where they are inverted, and the shapes of salt are turned out and left to dry. When the lumps made in the square tubs are dry, about 80-84 of them go to the ton.
When a pan of any of the boiled salts is to be started, the brine is run in, and some gelatine, glue, or sometimes blood, is added, while the fires are urged till the brine boils. A scum rises to the surface as the heating proceeds; this is removed, and the brine, from baying appeared at the first heating rather opalescent, becomes clearer, while the bottom of the pan gets whitened with an incrustation, particularly at first just over the fires. At the same time, if the brine is strong and good, there tends to form all over its surface a crystalline pellicle or crust. If fine salt is being made, this is broken dovvn by striking the water with a flat piece of wood from time to time ; but when the boiling commences, this pellicle breaks up as fast as it forms, and the floating bits of salt drift from the fire end of the pan towards the other, where they sink and accumulate. This opera tion goes on continuously, and the salt by this means falls down during the whole boiling in largest quantity at the end of the pan farthest from the fires, and when enough salt has accumulated to make it vvorth while, the pan is drawn. This is under ordinary circumstances about twice a day ; for butter-salt, perhaps a little less often. A pan of 33 ft. by 24 ft., which may be recommended as a convenient size for boiling, ought to yield 5-6 tons of salt per 24 hours, making 30-35 tons a week. But including stoppages for scaling and repairs, and taking into account the diminished yield when the pan gets very thickly encrusted, the real yield can hardly be reckoned at over 25 tons a week. The process is continuous, brine being run in as fast as the evaporation proceeds, the pan being thus always maintained about three-quarters full. The scale which forms on these pans is usually very thick, and contains a very large proportion of meohanically combined salt. This kind of scale is called " salt-scale," in contradistinction to that which forms on the unboiled pan, called " pan-scale " or " sand-scale," and which differs essentially from it in composition. Analyses of samples taken from the works of Thos. Ward and Messrs. Gibson will be found in Table V., p. 1733. Several inches of this scale vvill sometimes form in the course of a week in these boiled-salt pans, and, as may well be supposed, most seriously diminishes the evaporative duty of the coal. The fine and butter-salt pans are usually sealed once a week, aud require very frequent repairing. For this purpose, the fires are extinguished, and the pans emptied of their brine, which is usually run to waste, when workmen enter, break up and chip off the scale with picks, and shovel it away. Attempts have been made to do this scaling without emptying the pan, and at the Stoke works near Droitwich, this work is performed every two days by a man wading about with his feet in two wooden buckets. This is a decided advance on the method usually employed. The boiled-salt pans are liable to a sort of efflorescence of salt over their edges, which is cut off from time to time, and which, if not removed, would often siphon the brine to a consider ablo extent out of the pan by capillarity. These cuttings are sold for use in agriculture under the name of " claggings," and the salt-scale is at times ground and sold for the same purpose. The latter has also been employed in making coarse glass, and some is at present sold for fluxing purposes in metallurgical operations. It will be necessary to return to these boiled salts when considering machine and composite pans, and the Otto Pohl process.
Manufacture of TJuboiled Salts.—The various qualities, which only differ from one another in the size of the grain and their more or less perfect crystallization, depend for their production on the slow evaporation of the brine at temperatures far removed from boiling. They include the so-called " common" salt, the different qualities of " fibhery-salt," and that known as " bay-salt." Whereas the length of the pans in which the boiled salts are made is limited to 35 ft. at the outside, it being impossible to keep longer pans in a constant state of ebullition without such florco firing as would destroy them ; ou the other hand, in the manufacture of the coarser-grained uuboiled salts, the length of the pans is iuereased till all chalice of boiling is avoided, while a greater economy of fuel is attained. When a pan of cold brine begins to be fired, that part of the brine immediately over the fires naturally first takes the heat, aud, growing lighter by expansion, rises to the surface, the colder brine from the further part of the pan running in from below to take its place ; the warm brine, then gradually diffusing itself on the surface, goes to the far end Of the pan, where, cooling by evaporation or by contact with the other cold brine, it sinks, returning tu the hot end of the pan to become onco more heated. Thus is established a circulatory movement iu the brine,
supercate.nt eurrent of warm liquid flowing from the fire-end, and it cooler current alwe.ys flowing back below; this continues so long as the brine is kept only gently heated and is not aetually boiled. As soon as any excess of water the brine may have contained has become dissipated, the salt beg-ins to form, producing the "hopper-crystals" already mentioned ; while a crystalline crust collects on the surface, and drifts towards tbe further end of the pan, there sinking to the bottom. To avoid too much salt thus drifting to tho far end of the pan and filling that part too rapidly, thin narrow laths of wood are stretched across the surface of the brine, which stop these floating crystals and cause them to fall down. When a pan is first set down, it is customary to add 2-12 lb. of alum for a pan three-quarters full of brine. Alum is partieularly used in this way in making fishery-salt. The brine is first made to boil, or very nearly so; the fires are then damped down a little, and the temperature is thus maintained and the evaporation allowed to proeeed at a lower point, according to the quality of salt required. About 180° F. (224° F. being the temperature at which saturated brine boils) is the temperature for common salt. On the first heating, a seum rises tu the surface, as in the case of the boiled salts. This scum is mostly due to the alum, which is decomposed, and the alumina cotnes to the surface, carrying with it any suspended insoluble rnattors, and perhaps taking up organic impurities the brine may contain. As previously stated, most brines (those of Cheshire forni no exception) are saturated solutions of calcium sulphate, as well as of sodium chloride; and calcium sulphate is one of the few salts known to be lass soluble in hot than in cold water. The consequence is that the brine, besides containing some trilling particles of suspended matter, becomes more or less clouded by a deposit of calcium sulphate, possibly also by a little calcium carbonate when Bret strongly heated. This eithor falls down as incrustation, Or rises to the surface in the scum, and thus the liquor, which had become a trifle turbid, again clears. Alum is also occasionally, but not so often, used in this way for butter-salt, and at times, in the case of boiled salts, a small lump of butter or grease is addi ; this has a tendency to break up and throw down the crystals. These are among the supposed secrets of the sal t-intikers, s.nd the substances thus added are termed by them " poisons." The scale from unboiled pans differs considerably in composition from that formed in the boiled pans, as shown by tho annexed table :— To fill a pan with salt, the briue being allowed to flow in a small gentle stream in quantity sufficient to replace that lost by evaporation, takes, in the case of common salt, working at a tem perature of 71°-82° (160°-180° F.), 48 hours ; common or Scotch fishery salt, working at 54^-71° (130°-160° F.), 4-5 days ; extra fishery, working at 38°-43° (100°-110° F.) and in long pans, 7-8 days ; double-extra fishery, working at about 32° (90° F.), 10-14 days ; large-graiued bay-salt, working at 21°-27° (75°-80° F.), 3-1 weeks. The " wych-house," as the wooden shed is called in which the evaporation is carried on, is 15-18 ft. wider than the pan itself ; on each side of it, abut ting against its walls, are low shelves of boards, placed near the ground, sloping gently inwards, and running the entire length of the pan, though separated from it by a pathway in which the workmen can circulate. On to this shelf the salt is thrown to drain, one workman drawing it to the side of the pan with a rake, while another fishes it out with a scoop or skimmer. The men who work at the drawing of the pans are called in Cheshire " wallers," and the boards on to which it is thrown are called " hurdles." These latter (Figs. 1208-9) are 5-6 ft. wide. The salt is taken from them in wheelbarrows or trucks to the store by men termed " lofters." In England, it is generally managed, if possible, to place the wych-houses on a higher level than the store, so that the salt when wheeled there may be tipped from above into the bins reserved for each kind ; but in France, the construction of a salt-works to meet the requirements of the excise will rarely permit of this, so the pans and the bins into which the salt is tipped are put on about the same level, the salt being lifted from the hurdles to the tops of the covers (hottes) of the pans, on which it is dried before storage, instead of being merely drained on the hurdles as in England. This mode of treatment depends on the heavy tax (125 fr. or 5/. a ton) levied on all salt for domestic purposes in France, chemical and agricultural salts being alone exempted. Two excisemen are attached to and obliged to live in every French salt-works. while the works are compulsorily surrounded completely by a wooden paIisade about 7 ft. high and reaching the ground, the laths of which must not be over in. apart. There must be likewise but one gateway to the works. The result of drying the salt before placing it in the bins is that these French salts nearly always gain rather than lose weight transpoit, attracting more or less moisture from the atmosphere ; and being always in sacks, the purchaser feels that even If he has to pay a heavy tax, he gets good weight for his money, while the salt-maker pays his tax on tho lowest weight possible. The tax in France has likewise tended generally to maks the manufacturers (saulniers) careful to produce the finest and purest article prac ticable. They have likewise protected themselves by syndicates in the respective districts, so 0.13 tO ensure against over-production, and they have thus succeeded in maintaining prices at remu nerative rates. Further, as the tax is often only made payable 4-5 months after the salt has left the works, though sales are effected at short dates of payment, large sums constantly lie in tho hands of the manufacturers, and more than suffice to form their floatiag capital.